I struggled at the start of my career to "tell off" pupils (what language to use, etc). What really helped me was to have a colleague do that within earshot (or sometimes she'd come in and rollock my class). It helped me build up a bank of phrases and expressions, as well as imitate her tone of voice.
At the end of the day, if he/she isn't willing to improve it, it's going to be an uphill battle, because they'll apply the techniques only when observed, and then get back in their comfort zone the rest of the time.